Hi, I'm creating a fixed-layout picture book for KF. I have it completely built (html for each spread; images in their folder; embedded fonts, content.opf, css style sheet, etc)

I discovered that I can create a mobi by opening a complete content.opf (manifest lists all elements; spine gives play order; correct metadata for fixed-layout, etc.) through Kindle Previewer. It looks good on the Fire; my question is: is this a 'real' mobi? Or is this too easy, and I need to do something more robust? Thanks very much.

It looks good on the Fire; my question is: is this a 'real' mobi? Or is this too easy, and I need to do something more robust

The lastest version of Kindle Previewer automatically runs Kindlegen 2 in the background to generate a hybrid .mobi file that contains both a regular .mobi file and a KF8 file.
Why don't you simply open the .mobi file with your Kindle 3 to test it?
(If you want to create a version that will definitely work on older devices, compile your book with Kindlegen 1 or Mobipocket Creator.)

Thanks...this is a fixed-layout that will be sold only to Fire users, so doesn't matter if it works on K3 (this is what my client is telling me). I will put it on my K3 just to see what it looks like there.....

I was just wondering if this is a full Mobi, coming only out of the single .opf file (as opposed to the entire folder containing the whole job, or an ePub, which I'm accustomed to).

If it's made with the latest Kindlegen/KindlePreviewer, it will be a dual purpose mobi file: a mobi 6/7 part to work with older kindles and a KF8 part to work with Fires (and the very latest apps supporting KF8). It will also contain the all the source documents (open the final product in a zip utility to see those) used to create it.

Most likely—since you were concerned only with creating a fixed layout ebook for the Fire when designing it—it will still open on a K3, but will probably look like utter crap on anything NOT a Fire.

I did put it into K3 and, as you suspected, utter crap. Brought in pages that are all art, but dropped art on text/art combo pages. The text did retain indents and breaks, though. Doesn't matter, as not made for anything but the Fire.